1. Field of the Invention
This invention relates to a magnetoresistance element for reading a magnetic field strength associated with magnetic recording media and the like as a signal, especially a magnetoresistance element capable of reading a small magnetic field change as a significant electric resistance change signal, a magnetoresistance multilayer film suitable therefor, and a method for preparing the magnetoresistance element.
2. Prior Art
Recently there are growing demands for increased sensitivity of magnetic sensors and increased density of magnetic recording. Researchers strive for the development of magnetoresistance effect type magnetic sensors (simply referred to as MR sensors, hereinafter) and magnetoresistance effect type magnetic heads (simply referred to as MR heads, hereinafter) utilizing magnetoresistance changes. Both MR sensors and MR heads are designed to detect a resistance change of a reading sensor section using a magnetic material for reading an external magnetic field signal. Since the MR sensors and MR heads produce outputs which do not depend on their speed relative to magnetic recording media, the MR sensors have the advantage of high sensitivity and the MR heads have the advantage of high outputs even in high density magnetic recording.
Conventional MR sensors based on anisotropic magnetoresistance effect using magnetic materials such as Ni.sub.0.8 FeO.sub.2 (Permalloy) and NiCo, however, are short of sensitivity as MR head material for reading ultrahigh density record of the order of several GBPI since their resistance change ratio .DELTA.R/R is as low as 2 to 5% at most.
Attention is now paid to artificial superlattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked since their behavior is different from bulk metal. One of such artificial superlattices is a magnetic multi-layer film having ferromagnetic metal thin films and anti-ferromagnetic metal thin films alternately deposited on a substrate. Heretofore known are magnetic multilayer films of iron-chromium, nickel-chromium, and iron-manganese types (see Japanese Patent Application Kokai (JP-A) No. 189906/1985). Among these, the iron-chromium (Fe/Cr) type was reported to exhibit a magnetoresistance change in excess of 40% at cryogenic temperature (4.2K) (see Phys. Rev. Lett., Vol. 61, page 2472, 1988). This artificial superlattice magnetic multilayer film, however, is not commercially applicable as such because the external magnetic field at which a maximum resistance change occurs (that is, operating magnetic field strength) is as high as ten to several tens of kilooersted (kOe). Additionally, there have been proposed artificial superlattice magnetic multilayer films of Co/Cu and Co/Ag, which require too high operating magnetic field strength.
Under these circumstances, there was proposed a ternary artificial superlattice magnetic multilayer film having two types of magnetic layers having different coercive forces deposited with a non-magnetic layer interposed therebetween and exhibiting a giant MR change due to induced ferrimagnetism. For example, Japanese Patent Application No. 78824/1991 we filed prior to the present application proposes a structure wherein a nonmagnetic layer intervenes between two adjacent magnetic thin films having different values of Hc, the respective layers having a thickness of up to 200 .ANG.. Also the following articles are known.
(a) T. Shinjo and H. Yamamoto, Journal of the Physical Society of Japan, Vol. 59 (1990), page 3061
A structure of [Co(30)/Cu(50)/NiFe(30)/Cu(50)]x15 wherein numerals in parentheses represent the thickness in angstrom of the associated layers and the numeral after x is the number of recurring units (the same applies hereinafter) produced an MR ratio of 9.9% at an applied magnetic field of 3 kOe and about 8.5% at 500 Oe.
(b) H. Yamamoto, Y. Okuyama, H. Dohnomae and T. Shinjo, Journal of Magnetism and Magnetic Materials, Vol. 99 (1991), page 243.
In addition to (a), this article discusses the results of structural analysis, changes with temperature of MR ratio and resistivity, changes with the angle of external magnetic field, a minor loop of MR curve, dependency on stacking number, dependency on Cu layer thickness, and changes of magnetization curve.
These ternary artificial superlattice magnetic multilayer films are inferior in the magnitude of MR ratio as compared with Fe/Cr, Co/Cu and Co/Ag, but show a giant MR ratio of about 10% under an applied magnetic field of less than several hundreds of oersted. The disclosure of these articles refers to only MR changes under applied magnetic fields of about several 10 to 100 Oe.
MR change curves under an applied magnetic field of 0 to 40 or 50 Oe are important for MR head materials which are used in practical ultrahigh density magnetic recording. In the above-mentioned conventional ternary artificial superlattices, however, the MR change at an applied magnetic field of 0 is not so increased and it is approximate to 0. A percent increase of MR change reaches maximum at about 60 Oe, marking an MR ratio of about 9%. That is, the MR change curve has a slow rise. Permalloy (NiFe), on the other hand, is not suitable as MR heads for reading ultrahigh density magnetic recording because a slope of MR change at zero magnetic field is approximately 0, the MR ratio remains substantially unchanged, a differential of the MR ratio is approximate to 0, and magnetic field sensitivity is low.
One measure for solving such performance problems is proposed in JP-A 318515/1994 as a "magnetoresistance element comprising a magnetoresistance film having alternately deposited ferromagnetic layers and non-ferromagnetic layers, characterized in that the interface between a ferromagnetic layer and an adjacent non-ferromagnetic layer is inclined relative to the surface of the magnetoresistance film and electric current flows through said magnetoresistance film across said interface."
The magnetoresistance element proposed in the above-cited patent reference, however, is very difficult to manufacture and thus impractical since the magnetoresistance film of multilayer structure must be manufactured in the following manner, for example.
First, using a sputtering method, cobalt (Co) layers of 15 .ANG. thick and copper (Cu) layers of 9 .ANG. thick are alternately deposited 30 cycles on an (upper) surface of a silicon substrate ready for deposition. This results in a magnetoresistance film of multilayer structure on the substrate.
Next, a film of novolak type photoresist is formed on the surface of the magnetoresistance film and the resist film is heat softened to form a tapered mask. This mask gradually decreases its thickness from the right end to the left end of the film (as viewed in the figure attached to the above-cited patent reference, the same applies hereinafter).
Subsequently, the magnetoresistance film is subject to ion milling by irradiating argon ion Ar.sup.+ from above the tapered mask. As a result, the film is removed more on the left side than on the right side. The residual film is 10 .mu.m as a whole and its thickness, which reflects the shape of the mask, decreases from the right end to the left end.
On the surface of the tapered magnetoresistance film thus obtained, an alumina (Al.sub.2 O.sub.3) film of 0.1 mm thick serving as a support is deposited by CVD and other techniques. The support is formed on the graded surface of the film (which newly becomes a first film surface) to a substantially uniform thickness.
Next, a film of novolak type photoresist is formed on the back (lower) surface of the silicon substrate, and the resist film is heat softened to form a tapered mask (not shown) as above. This mask has a cross-sectional shape which is tapered such that the thickness decreases from the left end to the right end as opposed to the mask formed on the surface (upper surface) of the silicon substrate. Both the masks have the same degree of taper.
Subsequently, the substrate and the magnetoresistance film are subject to ion milling by irradiating argon ion Ar.sup.+ from below the mask on the back surface of the substrate. As a result, the substrate is completely removed. The film is removed in a tapered shape reflecting the mask shape. The graded surface of the film thus formed newly becomes a second film surface. In this way, a magnetoresistance film having parallel first and second film surfaces and a uniform thickness is obtained in a state secured to the support.
As mentioned above, the preparation of the magnetoresistance film in the magnetoresistance element proposed in the above-cited patent reference is impractical because many steps are involved, for example, ion milling using a mask must be done two times, and difficult operation is concomitant. The magnetoresistance element proposed in the above-cited patent reference wherein the interface between a ferromagnetic layer and an adjacent non-ferromagnetic layer is inclined relative to the surface of the magnetoresistance film also has the problem that a considerable increase of a vertical component of conduction electrons at the interface is not expectable since the angle of inclination cannot be increased beyond about 30.degree. for manufacturing and other reasons.